1. Field of the Invention
This invention relates to a cylindrical magnetron sputtering system and, more particularly, to a cylindrical post magnetron sputtering system especially suitable for deposition of thin films of certain compounds and alloys.
2. DESCRIPIION OF RELATED ART
Thin layers of conducting materials such as metals, metal silicides, and low-resistivity pclycrystalline silicon; and insulating substances such as silicon dioxide, silicon nitride, and phosphosilicate glass, constitute important elements of many semiconductor devices. Additionally, certain newly developed compoumds and alloys that can function as superconducting materials may become widely used commercially in the form of thin film structures.
Various techniques have been developed to produce such layers as part of a device. One such technique, known as film deposition involves supplying component materials for a growing layer from external sources and depositing those materials down upon a sutstrate. Such deposition processes are generally carried out in a vapor phase within a reduced pressure atmosphere of a selected gas or gases, or in a vacuum. If the material to be deposited does not react chemically during deposition, the process is referred to as Physical Vapor Deposition or PVD. If, on the other hand, the deposited material is a prcduct of a chemical reaction which occurs within the vapor phase, either on the surface or in the vicinity of the substrate, the process is known as Chexical Vapor Deposition or CVD. Hybrid methods of film deposition, i.e., those which involve both physical and chemical processes, are also known.
One method of physically depositing a film upon a substrate is known as sputtering. A typical sputtering system includes a target (a cathode) and a sutstrate holder (an anode) positioned so that the surface of a substrate upon which the film to be deposited which substrate is placed on the holder, faces the target. The target is a plate of the material to be deposited on the substrate or from which such a film is to be synthesized. The target is connected to a negative voltage supply, either dc or rf, and the sutstrate holder may be either grounded, floating, or biased, as well as either heated, cooled or some combination thereof. A gas, typically at a pressure from a few millitorr to about 100 mTorr, is introduced into a chamber containing the substrate holder and target to provide a medium in which a glow discharge plasma can be initiated and maintained. When the glow discharge is started, positive ions strike the target and stimulate the removal of mainly neutral target atoms therefrom by momentum transfer. These atoms then condense into a thin film formed upon the surface of the substrate placed on the substrate holder. In addition, various particles other than neutral atoms, e.g., electrons and ions, are also produced at the target which may have a significant effect on the properties of the film deposited on the substrate. It is very common for sputtering systems such as those generally described above to have planar cathodes positioned parallel to their respective anodes. Such systems are called planar magnetron sputtering systems.
Heretofore, efforts have been made to deposit various compounds and alloys in uniform films, reproducing the stoichiometry of the source material, using planar magnetrons. For example, for various reasons it has become desirable to be able to deposit thin filas of such compounds and alloys as Nb.sub.3 Sn, YBa.sub.2 Cu.sub.3 O.sub.x ("YBCO"), Bi.sub.2 CaSr.sub.2 Cu.sub.2 O.sub.x, and Bi.sub.2 Ca.sub.2 Sr.sub.2 Cu.sub.3 O.sub.x B"BCSCO") on substrates. In particular, it has become desirable to be able to deposit such compounds and alloys on substrates of a multitude of different sizes.
The above-mentioned efforts to deposit the various listed compounds and alloys using planar magnetrons have revealed a number of significant limitations. At low pressures, for example, virtually no deposit has been effected in the center of certain circular substrates because of erosion caused by negative oxygen ion resputtering. Additionally, analysis of film composition of films deposited at low pressures has revealed undesirable stoichiometric variations. At high pressures, somewhat less negative oxygen ion erosion has been found at the center of substrates subjected to deposition and film composition stoichiometry has been somewhat better than that at low pressure. However, material depcsited on certain areas of the substrates have been found to heavily contaminated because of exposure to the atomsphere. Additionally, atomic percentages of certain elements in targets after sputtering at high pressure have not been found not to be uniform; that is, the various compounds appear to depart the target leaving certain elements "rich" in certain areas and those same certain elements "poor" in other areas. Further details regarding these findings are set forth in the detailed description section below.
In addition to the limitations of planar magnetron sputtering systems described above, other problems and shortcomings of such systems have heretofore been identified. Examining the sputtering process employed in a conventional planar diode sputtering system in more detail, a low pressure abnormal negative glow plasma discharge is maintained within a chamber between a planar cathode (target) and a planar anode (substrate holder). Electrons emitted from the cathode due to ion bombardment thereof are accelerated to near the full applied potential within the cathode dark space, i.e., a relatively nonluminous region between the cathode and the negative glow. Such high energy electrons enter the negative glow as so-called primary electrons where they collide with gas atoms and produce the ions required to sustain the plasma discharge. The primary electron mean free path increases with both increasing electron energy and decreasing pressure within the chamber. At low pressures, ions are produced far from the cathode where their chances of being lost are great. Additionally, many primary electrons hit the anode with high energies, causing a loss that is not offset by impact-induced secondary emission. Thus, ionization efficiencies are low. As the pressure within the sputtering chamber is increased at a fixed voltage, the primary electron mean free path decreases and larger currents are possible; however, at high pressures within the chamber the sputtered atom transport which occurs has been found to be reduced by collisional scattering.
lt has also been found that a magnetic field extending parallel to the cathode surface can restrain primary electron motion to regions in the vicinity of the cathode and thereby increase ionization efficiency. It has teen further found that the E x B eleotron drift ourrents can be caused to close on themselves by the use of cylindrical cathodes, which thereby prevent the E x B motion from causing the plasma discharge to be swept to one side. Based upon the foregoing, various cylindrical magnetron systems have been developed. Such systems having spool-shaped cathodes are known as cylindrical post magnetrons. As the adjective "spool-shaped" implies, the cathodes in cylindrical post magnetrons comprise a cylindrical barrel and two disk-like wings. Circularly surrourding both of the wings are wing shrouds. Typically, one wing is made smaller than the other. Anode action of the shroud on the smaller wing acts to terminate uniform extension of plasma outward from the barrel portion of the cathode. In a typical design, the smallest wing size is three times larger, or more, than the gyro radius of the primary electron exitted from the cathode. Also, with respect to typical cylindrical post magnetrons, a solenoid placed coaxially and externally to the cathode is used as the field generator. Such solenoids are generally wound on a core of magnetic material.
With respect to use of cylindrical post magnetrons, heretofore one of the most significant uses has been deposition of photomask quality chromium on glass. Such magnetrons have also been used to produce iron oxide coatings by reactive sputtering. In general, cylindrical post mgnetrons have heretofore been used to coat very large articles and/or very complex-shaped articles.
Based upon the foregoing, it should be appreciated that efforts to deposit such alloys and compositions as Nb.sub.3 Sn, YBa.sub.2 Cu.sub.2 O.sub.x, Bi.sub.2 CaSr.sub.2 O.sub.x, and Bi.sub.2 Ca.sub.2 Sr.sub.2 Cu.sub.3 O.sub.x have not been wholly satisfactory. Planar magnetron sputtering systems have been the primary systems employed heretofore to attempt such deposition. Although other magnetron sputtering systems, such as cylindrical post magnetrons, are known, they have not heretofore been used or specially modified so as to be suitable for use in effecting deposition of high quality films of such materials as Nb.sub.3 Sn, Yba.sub.2 Cu.sub.3 O.sub.x, Bi.sub.2 CaSr.sub.2 O.sub.x and Bi.sub.2 Ca.sub.2 Sr.sub.2 Cu.sub.3 O.sub.x.